In the microelectronics industry there is a need for adhesives that can be processed quickly in addition to having the requisite electrical properties. Currently used die attach adhesives include organic based materials that employ epoxy resins, polyamic acids and polyimides. These are paste-like materials that may or may not contain solvents. All of these materials have significant disadvantages. They all require a bake cycle off-line to either cure the adhesive or remove the solvent. The epoxy resin and polyamic acid based products also crosslink during cure producing significant stress on the die that ultimately may cause die cracking. These materials also have to be refrigerated.
Die attach adhesive based on glyme or other ether solutions of thermoplastic resins, such as, siloxane-modified polyimides were developed to replace the more conventional die attach adhesives that use thermoset resins as a binder. In the fabrication of semiconductor devices, the adhesive is deposited onto the paddle of the lead frame, the die then placed on top of the adhesive and the solvent removed by means of heat. Although solvent-containing poly(siloxaneimide) based systems offer high speed processing, certain problems are encountered. First the rate of solvent evaporation must be carefully controlled. Prior to deposition on the die bond pad, the solvent must be retained by the paste system. Excessive solvent loss, prior to deposition of the die can lead to inadequate adhesion. After paste deposition and die placement, it is desirable to remove the solvent from the adhesive as quickly as possible. This is difficult because the bulk of the paste mass is covered by the die which interferes with the escape of solvent. Solvent retention by the adhesive can cause voids to develop under the die upon exposure to high temperatures which in turn can result in failure during thermal cycling due to localized areas of high stress.
With the advent of increased use of large dice (1".times.1" or larger) a die attach adhesive is needed that does not produce excesssive stress on the die because of crosslinking or thermal stress such as that resulting from excessively high cure temperatures. The former is associated with the curing of epoxy resins while both areas of stress buildup are associated with polyamic acid based systems. A solvent containing adhesive also presents problems with large dice because of the difficulty in removing all of the solvent as explained above. Metallic adhesives, such as gold and gold silicon eutectic performs, do not have these problems but still produce significant stress on the die because of the brittle nature of the metallic bond.
It is therefore an object of this invention to provide a die attach adhesive that bonds rapidly without the need for curing or baking off-line and does not produce excess stress on the die.
Another object is to provide an adhesive that need not be refrigerated.
Still another object is an adhesive that provides uniform bond line thickness between the die and paddle.
A further object is to provide an adhesive that can be used at temperatures that do not oxidize copper lead frames or produce excessive thermal stress.
It is also desired that the adhesive can be reworked and can exert adequate adhesion during the processes of wire bonding and molding.
Other objects will become apparent to those skilled in the art upon a further reading of the specification.